Lauren Davis

Click to viewMost of the the search for life on other planets has focused on finding a world just like ours: rocky and covered in liquid water. But as astronomers learn more about exoplanets, some are beginning to suspect that life can evolve under a wider range of conditions than anyone had thought before. Far from being unique, Earth could be just one world in a universe teeming with truly diverse breeds of life.Astronomers have long referred to regions of space where conditions are favorable for life as the “Goldilocks Zone.” Conventional wisdom is that Earth is a rarity, a world where conditions happen to be “just right” for sustaining life. But as they discover new worlds and run simulations on virtual planets, many astronomers are expanding their definition of habitability. For one thing, says David Spiegel of Princeton University, planets that experience harsh conditions may still have habitable zones or habitable seasons, which may be sufficient for life to survive at certain times:

Spiegel argues that this kind of simulation shows that astronomers should not think of habitability as an all-or-nothing thing. It makes more sense to think in terms of "fractional habitability", he says, as in what fraction of a planet's surface is habitable, for what fraction of the year, or for what fraction of its history. "Even the Earth is not 100 per cent habitable, at least by the standard liquid-water definition," Spiegel points out. "Parts of the planet are frozen part of the time. Parts of the planet are frozen all of the time."

Additionally, while Earth’s life-sustaining heat comes from its particular distance from the sun, other planets may reach an optimal temperature in other ways:

This year, a team led by Brian Jackson of the University of Arizona in Tucson explored the extent to which some planets have internal heat sources. Planets orbiting close to a star or with non-circular, eccentric orbits move towards and away from their star in the course of an orbit. As a result, they are stretched and squeezed by variations in the gravitational pull from their star, and this causes enough friction in their interiors to generate heat.

And all this assumes that life on other planets has similar climate requirements to those of life on Earth. Jonathan Lunine at the University of Arizona notes that we don’t yet fully understand why life has flourished on Earth and it would be presumptuous to assume all life requires the same conditions:

There is always the chance that the search for liquid water on the surface may be missing the point. What if exotic forms of life could thrive where there is no liquid water at all - swimming around in lakes of liquid methane on Saturn's frigid moon, Titan, for example? "One should not rule out the notion that a kind of life or organised chemistry could exist in that kind of liquid," says Lunine. "Let's cast the net broadly."